Despite the fact of a large number of works on wetting properties of Pb-containing standard solders and candidate lead-free solder alloys, there is a great uncertainty about the role of testing atmosphere and procedure on wetting behavior of these alloys and how these factors affect the correlation between solderability and mechanical properties of solder joints.

In this study, the wetting of Cu substrates by several Sn-based lead-free solder candidates has been studied by a sessile drop method in air using standard flux or in vacuum applying different testing procedures. The structure and chemistry of solidified and cross-sectioned solder/Cu couples were examined by optical and scanning electron microscopy. Mechanical properties of the same couples were determined by push off shear tests.

The analysis shows that there is no important difference in properties of selected solder/Cu couples obtained in vacuum and in air with flux. For situations when the same combination of solder/Cu couple under identical testing conditions (time and temperature) showed dissimilar wetting and mechanical behaviors, it was noted that either the solder or the Cu substrate had oxidized regions, responsible for a local dewetting and for the structural defects, all contributing to pure wetting and weakening of the couples.

This work was done under the group project GP7 entitled "Prediction of New Generation Solders" and realized in the framework of European Concerted Action on "Lead-free solder materials" (COST 531). The authors are grateful to the Ministry of Science and Information Technology of Poland and the Foundry Research Institute for financial support.